I was wondering if I could stop my current download (currently on Windows), start my Linux Distro (native installed) and mount the windows drive, then download there the game via wget or something similar. That would be great, because then I could do something on my linux while the game is downloading in the background and so I don't have to wait an eternity on windows where I can't do much (I do Kernel programming and such things)
I would guess Steam does not allow you to download games just via the command line.
There are loads of things I don't know how steam would deal with, like checking your licence to the game, is you were just to download a steam game via command line without having a steam install on your machine.
So I would say, before bashing your head on a problem like this, just go download Steam (theres a linux distribution) and download the game via it. There's no need in having a hard time for hours while you can just download the game you want it 2-3 clicks.
(Really at the end of the day I would guess that if you can download the game via command line it will just package steam into it if you don't have it anyways.)